The present invention relates to a semiconductor integrated circuit device such as a large scale integration (hereinafter called “LSI”) which performs power control to realize a reduction in power consumption.
In order to achieve a reduction in power consumption of a semiconductor integrated circuit device (e.g., LSI), a technique for bringing a power supply brought to an on (hereinafter called “ON”) state at a normal operation accompanied with the need for its operation to an off (hereinafter called “OFF”) state upon a waiting state (standby state) free of the demand for its operation has heretofore been described in, for example, patent documents (Japanese Unexamined Patent Publication No. Hei 5(1993)-259879 and Japanese Unexamined Patent Publication No. 2003-332901) or the like.
As described in these patent documents or the like, the LSI is not generally subjected to the demand for its operation at all times during applications and always repeats the state of the normal operation accompanied with the need for its operation and the standby state free of the demand for its operation. It is general that measures to reduce power consumption are taken in the standby state free of the need to allow the LSI to function.
As a power management method for reducing or suppressing power consumption of the LSI, attempts have heretofore been made to suppress a state transition of each transistor lying inside the LSI and thereby reduce an operating current, using a method for reducing the frequency of a clock signal for synchronization or stopping the clock signal.
However, the conventional measures to reduce power consumption are not capable of suppressing a quiescent current (leak current) peculiar to the transistor although the operating current can be reduced. With the growing speed of the LSI and an increase in the number of gates, there has recently been a tendency that a transistor having high drive capacity is frequently used. This type of transistor causes a large leak current. With its greater use, the leak current appears as a value so large as to be innegligible with respect to the total operating current of the LSI. Therefore, it is becoming hard to obtain the effect of reducing the power consumption where the operation transition of the transistor is merely suppressed.